Various medical procedures require the making of electrical contact with specified regions within the heart. For example, in an intracardiac ECG (electrocardiogram), an apparatus such as a catheter or a probe is inserted through a vein or an artery into the appropriate location within the heart. The apparatus has one or more electrodes which are placed in close proximity to the tissue of the heart so that the electrical activity within the heart can be appropriately monitored. Intracardiac ECG sensing can be done when the apparatus is in the heart for other purposes, such as pressure monitoring, measuring cardiac output or right heart ejection fraction.
Similarly, impedance measurements to determine physiological functions or parameters, such as for example, blood flow, chamber volumes and continuous cardiac output, require electrical contact between two or more electrodes and the body. For impedance measurements, electrodes are typically mounted on a catheter and inserted into the vascular system or the heart, or into other parts of the body depending upon where the impedance measurements are to be taken.
Temporary pacing of the heart also requires the making of electrical contact between one or more electrodes of an apparatus, such as a probe or catheter, and the tissue of the heart. For example, during certain surgeries a catheter may be inserted into the heart to monitor various cardiovascular functions, such as cardiac output or right heart ejection fraction. Such a catheter may be equipped with appropriate pacing electrodes so that, if the patient should suffer a cardiac arrest, the heart can be quickly given the necessary electrical therapy.
It is conventional practice to mount the electrodes on the exterior surface of a catheter, and constructions of this type are shown, for example, in Blake et al., U.S. Pat. No. 3,995,623 for a Multipurpose Flow-Directed Catheter, issued Dec. 7, 1976. Unfortunately, there is a danger that electrodes mounted on the exterior of the catheter may become loose and slide off the catheter and remain in the patient. This can occur, for example, as a result of sliding the catheter through a tubular introducer which is used in placement of the catheter. In addition, an electrode mounted on the exterior of the catheter tends to project radially beyond the catheter body line and, as such, is more likely to slide off the catheter as a result of relative movement between the catheter and the introducer.
U.S. Pat. No. 4,595,012, issued June 17, 1986, for Lumen Mounted Electrodes for Pacing and Intra-Cardiac ECG Sensing, in the name of Webler, et al., which is incorporated herein by reference in its entirety, sought to solve this problem by mounting a tubular metal electrode within a lumen of a catheter and by placing the mounting means for the electrode at least partially within the lumen. To make the dismounting of the electrode even less likely and to reduce the likelihood of the electrodes forming an impediment to movement of the apparatus, the electrode and the mounting means lie radially inwardly of the body line of the apparatus.
In Webler, the electrodes are tubular conductive members or sleeves of a suitable conductive biocompatible metal such as stainless steel. The mounting means includes a tubular, resiliently bendable mounting member of a suitable nontoxic material, such as polyvinylchloride. The cylindrical mounting member forms a sliding fit within the cylindrical axial passage of the tubular electrode. The mounting member extends completely through the electrode and has end portions in the lumen on opposite sides of the electrode that form a mechanical interlock with the catheter tube on opposite sides of the port opening. The mounting means also includes an insulating adhesive, such as urethane, which bonds the electrode to the mounting member and bonds both of these members within the lumen and to the catheter tube.
The use of a conductive polymer on the tip of a body implantable lead for pacing is known. See for example European Patent Application No. 0057450, published Nov. 8, 1982. The cardiac pacing lead disclosed in the European application comprises a coiled flexible conductor and an overlying sheath. The conductor is in electrical communication with a polymer gel electrode by means of a metal crimp-sleeve. The sheath provides electrical insulation for the conductor. The lead body defines a cavity at the distal end which contains the polymer electrode with a rounded end portion extending beyond the open distal end of the lead body. The rounded end provides electrical contact between living tissue and the conductor. The polymer electrode consists of a conductive polymer gel, preferably a hydrogel which extends out of the lead body for about 0.5 mm to about 1.0 mm.
The European application also discloses a second embodiment wherein a ring-electrode for a bipolar lead is provided and includes a second flexible conductor and a band of the polymer gel formed over the insulating sheath but in contact with the second conductor.
U.S. Pat. No. 4,198,991, entitled Cardiac Pacer Lead issued Apr. 22, 1980 in the name of Harris, discloses a cardiac pacer lead utilizing as a conductive element a thread of conductive carbon filaments in a resin matrix. The thread is helically wound between a stimulation electrode structure at the distal end of the lead and a terminal at the proximal end. To establish the electrical connection at each end of the conductive element, the carbon filaments are first bared and then bonded with a conductive, e.g., silver or platinum-loaded, epoxy adhesive to the metal electrode and terminal components. See Column 3, line 31-40.
U.S. Pat. No. 3,721,246, entitled Applicator Electrode with a Very Thin Non-Metallic Current Distributing Layer, issued Mar. 20, 1973 in the name of Landis, discloses a body contacting electrode. The electrode has a dry, current distributing, skin contacting layer selectively disposed adjacent a conductive portion to provide a high resistance electrode arranged to limit and uniformly disperse the current from an electrical source through the skin area adjacent the electrodes. The current distributing layer is comprised preferably of conductive particles such as carbon or the like, uniformly distributed in a nonconducting plastic medium such as epoxy or other insulating material. Means are provided to couple the electrode to an external electrical source and to releasably support the electrode adjacent a preselected area of the body.